1. Field of the Invention
The present invention relates generally to super hard, incompressible materials. More particularly, the invention is directed to the discovery that osmium, when combined with boron alone, or in combination with rhenium, ruthenium or iron, produces compounds that are ultra-hard and incompressible. These osmium diboride compounds are useful as a substitute to for other super or ultra-hard materials that are presently being used in cutting tools and as abrasives.
2. Description of Related Art
The publications and other reference materials referred to herein to describe the background of the invention and to provide additional details regarding its practice are hereby incorporated by reference. For convenience, the reference materials are numerically referenced and identified in the appended bibliography.
High bulk modulus, superhard materials are of great interest due to their usefulness in a wide variety of industrial applications. These include abrasives, cutting tools, and coatings where wear prevention, scratch resistance, surface durability and chemical stability are a priority.[1,2] Therefore, the development of a new class of superhard materials is of great practical interest. Past research suggests the general idea that melting points can be related to hardness and that the addition of carbon, nitrogen, or boron to transition metals will often form hard materials.[3]
Hardness is a measure of a material's resistance to plastic indentation under an applied load. The deformation of the material is manifested through shear deformation, volume compression, bond-bending, and dislocations.[4] A high shear modulus reflects the material's ability to undergo compression without deforming in any direction. This is related to the material's ability to resist volume compression and is reflected in a high bulk modulus.[5,6] Therefore, a design goal is to find a material that possesses a high bulk modulus.[7-9]
The compressibility of a wide variety of substances can be directly correlated with the densities of their valence electrons, electrons/Å3.[10] For example, diamond, the hardest known substance, has a valence electron density of 0.705 electrons/Å3 and an exceptionally high bulk modulus (B0=442 GPa).[11] Maximizing the valence electron density is therefore a potentially useful design parameter in the search for new high bulk modulus materials. Osmium metal has one of the highest valence electron densities for a pure metal (0.511 electrons/Å3) and recent measurements of its bulk modulus give values in the range of 411-462 GPa.[12,13] While one measurement actually suggests that osmium metal is less compressible than diamond, both experiments agree that it is a highly incompressible material. Although the bulk moduli of diamond and osmium are very close, their hardnesses vary greatly due to differences in the mobilities of their dislocations.[5] Diamond has a hardness of 8000-10,000 kg/mm2, while osmium metal has a hardness of only 400 kg/mm2.[14,15] The deviation can be explained simply: osmium is metallic, whereas diamond is purely covalent. The atomic orbitals in diamond are all sp3 hybridized, and overlap to form short, directional, highly covalent bonds in an infinite tetrahedral network. The strength and directionality of the bonds determine the material's ability to resist deformation.[4,8] In contrast, osmium's hexagonally close-packed crystal structure has a Fermi-liquid of valence electrons which do not participate in localized or directional bonding, and therefore offer little resistance to dislocation motion.
While many transition metals are soft in their pure elemental state, they can be converted into hard materials by combining them with small, covalent main group elements such as boron, carbon or nitrogen. For example, consider the third row transition metal tungsten. The hardness of tungsten is increased from 900 kg/mm2 to 1400 kg/mm2 by the addition of boron to form tungsten diboride, WB2. Alternatively, tungsten can be combined with carbon to form tungsten carbide, WC, which increases the hardness to 3000 kg/mm2.[14]